1. Field of the Invention
This invention generally relates to a laser module and a method of controlling wavelength of an external cavity laser.
2. Description of the Related Art
Recently, an information-processing device and so on using a semiconductor laser have been developed. There is a demand for superior characteristics such as tunability or stability of wavelength. And so, Japanese Patent Application Publication No. 2003-152275 (hereinafter referred to as Document 1) discloses an art in which a wavelength selectable etalon whose inclination is tunable is provided between a semiconductor optical amplifier and an external mirror.
In accordance with the art, peak wavelengths of a light passing through an etalon change when the inclination of the etalon is changed. And, an effective gain range of the semiconductor optical amplifier includes one of the peak wavelengths. Therefore, an output wavelength of the semiconductor optical amplifier is selected to the peak wavelength of the light passing through the etalon.
However, the art disclosed in Document 1 has a problem that a full width of half maximum (FWHM) value of the peak of the light passing through the etalon is broadened when a Free Spectral Range (FSR) of the etalon is broadened so that the effective gain range of the semiconductor optical amplifier includes only one of the peak wavelengths of the light passing through the etalon. In contrast, the effective gain range of the semiconductor optical amplifier includes more than one peak of the light passing through the etalon when the FSR is reduced in order to reduce the half-value-width of the peak of the light passing through the etalon. Therefore, the selection accuracy of output wavelength of the external cavity laser is degraded.